Geek Answers: Why do some foods last forever?

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As an archaeologist, you don’t get many opportunities to sample the delicacies of the ancient cultures you study. Even wine turns sour and undrinkable with time — despite that 400 BC was a “good year.” Yet, there are a few foods upon which a hungry tomb raider might hope to stumble, foods that remain edible no matter how long they’re left out. For example a people might have figured out how to distill alcohol to high-percentage spirits or to fully dry out rice. Pots of honey have been found in ancient Egyptian tombs, still sweet and safe for eating. So why do some foods — both processed and all-natural — seem to resist spoiling so effectively?

We’ll disregard simple ingredients like “salt” from this explanation, since asking why salt never goes bad is a bit like asking why granite never goes bad; it’s a rock, man, and we just happen to eat it. The reason some actual foods never spoilcan basically be boiled down to this: they are inhospitable to life. While a hardcore digestive system (like the stomach) can envelop and break down a small amount of honey or strong alcohol, the bacteria, fungi, and insects that do most of the world’s decomposing need a foothold to get their job started. While evolution has created decomposers suited to virtually every niche there is, it’s been simply too difficult to be worth figuring out how to break down the world’s least digestible foods.

Some forms of vinegar will keep forever.

Sometimes the impediment comes in the form of extreme environments. Ethanol (the alcohol in our beverages) is a toxic substance, and a concentrated alcoholic beverage will burn right through any tiny organism unfortunate enough to enter it. Even the moderate levels of alcohol found in beer and wine will keep decomposers at bay for a while — which is probably why fermentation was developed in the first place. Preservation via alcohol wasn’t perfect until the invention of distilleries — but you can’t use whiskey as your main source of hydration, anyway. Extreme pH levels play a similar role in preservation, as with vinegar and, yes, even honey.

Though it might seem thick and neutral, honey is actually about as acidic as wine with a pH of 4 or below. The reason it stays good so effectively, though, is that it combines this moderate acidity defense with a much more extreme one: it’s very, very dry. Fermentation is one method of preserving perishable foods, but salting is another: we use salt to pull all available moisture out of the food, and since all life requires water this makes the situation untenable for any would-be attackers.

With our big saliva glands we can hydrate food as we eat it — but microorganisms? They so much as touch the surface of a moisture-sucking jerky and they find themselves suddenly dessicated, drained dry of moisture and left to blow away like a tumble-weed. Salted foods will eventually spoil, but not for a very, very long time.

It might look raw, but ancient processing techniques can keep this meat good for a long time… and make it delicious.

Honey might seem like an odd food to call “dry” since it runs like a thick liquid, but that ooze is a matrix of long, complex sugar molecules. These molecules are “hygroscopic,” which means they have an extremely strong affinity for water. They actually have very little water available, and when a microorganism tries to digest all that densely packed sugar energy it finds an environment similar to that in salted foods — dry beyond belief. Drained of all its moisture, all would-be decomposers are incapable of getting a foothold.

Honey even has a third defense, a minute ability to emit hydrogen peroxide — this contaminant to bacteria both keeps the honey good and helps humans keep wounds clear of infections. Unlike much ancient medicine, honey-coated bandages actually do have some basis in science.

Some foods remain unspoiled because they’re simply too toxic, others because they’re too dry and inhospitable, and others because they’re simply too hard to break down. Dried rice is less of a water-thief than some other foods, but its energy is packed away so tightly that microscopic life has yet to find an effective inroad to digestion.

This is the common thread to all foods with infinite shelf-lives: they are simply too inhospitable to life. Think about that, next time you and your friends and downing shots of aged whisky.